Cloak of Shadows

[thejeff]

But so what?

How will you abuse it?

(Especially an artifact, so you can only get one.)

So you've got a Tartarian Assassin. What will you do with him that'll "abuse the crap out of it"? With 2 slots filled, he's going to be more vulnerable than an equivalent non-Assassin SC.
You can only assassinate once a turn, more likely every other turn to avoid getting caught by patrolling armies designed to stop him. (And if your Assassin can take out the patrolling armies, why are you assassinating and not just conquering.)
And since targets are picked randomly, most of the time you'll be killing low-level mages and cheap commander chaff.

It'd be a useful tool, but I don't see how it'd be game-breaking.
Maybe if you could spam Tartarian Assassins.